This invention relates generally to demountable printing cylinders, and more particularly to a mandrel assembly for supporting a demountable cylinder and including a pair of hydraulically-actuated sleeves which are expansible to engage the end heads of the cylinder to lock the cylinder to the mandrel assembly.
In gravure printing, use is made of a printing cylinder whose surface is etched with cup-like cells which, as the cylinder passes through an ink fountain, pick up and carry the ink. When the cylinder engages an impression roller, the ink is transferred to the surface of the paper running therebetween. Flexographic printing uses similar inks, but the ink is picked up by rubber printing plates attached to a cylinder.
Since in the course of such printing operations, it is frequently necessary to replace one cylinder by another, various expedients have heretofore been proposed to provide demountable cylinder structures whereby the same mandrel may be coupled to different cylinders for use in the printing machine.
The simplest mechanical expedient for this purpose is set-screws to attach a cylinder to the mandrel. While set-screw arrangements are uncomplicated, they have many serious practical drawbacks. It is difficult to achieve proper concentricity with set screws; and as a consequence, the printing is of poor quality. Moreover, set-screws tend to vibrate and work loose. Other more complicated mechanical locking devices, such as split-lock clamping collars and expanding collets, have been suggested, but these are generally more expensive and equally inaccurate.
One may obtain accurate mounting for printing cylinders using a heat-shrinkage procedure to attach and detach a cylinder to or from a mandral. This procedure involves end closures on the cylinder having a relatively high coefficient of thermal expansion with respect to the mandrel, and it requires special heating equipment. Not only is the procedure time-consuming, but should axial or side-to-side adjustment of the cylinder on the mandrel be necessary, the heating procedure must be repeated with a further loss of time.
Another known approach makes use of hydraulically-actuated collet locks for demountable cylinders. However, known devices of this type require grease guns to pump fluid into the lock each time a locking action is to be effected, the grease being bled off each time the mandrel is to be released. The use of grease in the enviornment of printing operations is obviously undesirable. Moreover, it is not possible with such known devices to determine, without the use of additional expedients, the amount of hydraulic pressure that is being imposed on the mandrel, and whether it is sufficient to afford adequate torque resistance. As a consequence, cylinder creep or slippage may be encountered in the course of printing, with deleterious effect.
The Hoexter patent 3,378,902 discloses a printing cylinder having a pair of hydraulically-actuated collets mounted at opposing ends thereof, the mandrel for supporting the cylinder being slidably receivable within the collets and securely locked thereto when hydraulic pressure is applied. Each collet includes a cylindrical sleeve having a thick-walled hub section and a relatively long thin-walled pressure section. A broad circumferential channel is cut in the pressure section to form a bendable pressure wall, the pressure section being surrounded by a collar of high tensile strength whose edges are welded to the pressure section to define an annular fluid chamber bounded by the collar and the pressure wall.
A pressure cartridge is fitted into a cavity in the hub section, the cartridge communicating with the fluid chamber in the pressure section and including a piston which is advanced inwardly by an adjusting screw. When the annular chamber is filled with hydraulic fluid and the piston is advanced inwardly, the resultant hydraulic pressure causes flexure of the pressure wall, thereby subjecting the mandrel to radially-directed stresses which are uniformly distributed and serve to lock the mandrel to the collet and at the same time to maintain proper concentricity.
In the Hoexter arrangement, the mandrel is a standard shaft, but the demountable cylinder is not of standard design. It is a special cylinder which includes a pair of hydraulically-actuated end collets, as described above. Hence the special cylinder is substantially more expensive to manufacture than a standard cylinder. Since each machine in the printing facility is provided with several special cylinders each operable with a common mandrel, the overall cost of this arrangement is high.